Geotechnical information of the seafloor is often needed for proper engineering design of structures such as fixed leg jacket structures, tension leg platforms, spread moorings, gravity based structures and pipelines. The cone penetrometer is an in situ testing tool that can be used to perform cone penetrometer test (“CPT”) to gather geotechnical engineering properties of seafloor. For most offshore applications, a large deployment system is needed to deliver the cone penetrometer to the seafloor. Typically, the cone penetrometer gathers data as its cone shaped tip is pushed into the soil at a near static or static rate of speed. The standard push velocity is ˜2 cm/sec (±25%) according to industry accepted American Society for Testing and Material (ASTM) protocol. Readings are taken continuously every 1 cm to 5 cm or so to obtain continuously sampled static data. The length of the cone rod determines depth of push and varies typically from about 1.5 m to 4.5 m depending upon which system or specific tool is employed. In general, static CPT requires large and expensive equipment that can provide a stable platform at the seabed. Utilized from the stable platform, the cone penetrometer can then be inserted with a steady pressure at a controlled rate.
Various tools have been developed to deploy cone penetrometers in offshore environments. For deep-water investigations, a cone penetrometer can be operated in conjunction with wire-line drilling techniques with equipment mounted on a large drill vessel. Since the cone penetrometer is pushed at a constant rate, any drill string that secures the cone penetrometer to the vessel must remain immobilized so that the tool is essentially unaffected by vessel motion during the push. Immobilization of the drill string can be accomplished using a weighted seabed frame (SBF) that is designed to allow the drill string to be attached to the heavy weighted seabed frame (e.g., ˜20,000 lbs). The SBF is normally lowered to the seabed prior to spudding a borehole from a large winch on the deck of the vessel. The SBF is lowered through a large center well through the vessel. The drill rig is usually positioned over the large center well. Drilling heave compensators are used for both the drillstring and SBF to reduce influence of sea waves. When the drill string is at a desired depth, hydraulic rams on the SBF are activated and clamp onto the drill string. Once the clamps grip the drill pipe firmly, weight of the SBF is added onto the drill string and allows the drill pipe to be essentially motionless (since it is now tied to the seafloor). The added weight of the SBF on the drill pipe provides the heave compensators with enough resistance to allow the drill string and SBF to remain motionless during the insertion of the cone penetrometer into the seabed.
A recently developed offshore cone penetrometer tool is “allowed to free fall” into the seafloor to gather both static and dynamic CPT data. As used herein, the term “static CPT data” refers to CPT data collected when a cone penetrator is pushed at a static rate (typically at ˜2 cm/s). As used herein, the term “dynamic CPT data” refers to collection of CPT data at a non-static rate (much faster than 2 cm/s). An example of an offshore cone penetrometer system was described in a paper entitled “‘CPT Stinger’—An Innovative Method to Obtain CPT Data for Integrated Geoscience Studies” presented at Offshore Technology Conference (May 2-5, 2011).
In this offshore cone penetrometer system, the cone sensor portion is installed using a large piston corer weight-head and allowed to free-fall and penetrate into the sediment to about 20 m. During this time, dynamic CPT data is gathered. Once the offshore cone penetrometer tool is embedded, the cone tip can be pushed down to about 40 m at a static push rate (˜2 cm/s). The offshore cone penetrometer tool is designed to quickly assess soil properties by converting the dynamic CPT data to static CPT data using velocity algorithms. One of the main drawbacks of the offshore cone penetrometer tool is that the tool requires the use of a large seabed frame and heave compensator system. One primary limitation of the Stinger CPT tool is that it cannot measure CPT data beyond ˜40 m (˜20 m of dynamic data and ˜20 m of static data).